/Engine/Source/Runtime/Engine/Public/MaterialCompiler.h
Casts the passed in code to DestType, or generates a compile error if the cast is not valid. This will truncate a type (float4 -> float3) but not add components (float2 -> float3), however a float1 can be cast to any float type by replication.
int32 ValidCast ( int32 Code, EMaterialValueType DestType )